The present invention relates generally to gas furnaces for residential and commercial heating systems. Specifically, the invention relates to leak detection apparatuses and methods for inspecting heat exchangers of natural gas furnaces.
Gas furnaces comprise two circulating air systems: (1) the living space air circulating system which passes living space air over the exterior of the heat exchanger, and (2) the gas exhaust circulating system which passes hot combustion products through the interior of the heat exchanger. To ensure no harmful products of gas combustion escape from the interior of the heat exchanger into the living space air circulating system which passes over the exterior of the heat exchanger, it is important to determine whether holes or cracks have developed in the heat exchanger.
As shown in FIGS. 1A and 1B, the living space air circulating system 1 is described relative to a typical gas furnace 3. Air is circulated from the living space of the residential or commercial structure through the intake port 4. A turbine, not shown, pulls air into the intake port 4 and pushes it toward the heat exchanging section 5 of the living space air circulating system 1. During operation, the interior of the heat exchanger 7 is warmer than the air pulled from the living space. Thus, as the living space air passes over the exterior of the heat exchanger 7, heat is transferred from the heat exchanger 7 to the living space air. Finally, the warmed living space air passes from the heat exchanging section 5 through the plenum 6 for distribution back into the living space of the structure.
As shown in FIGS. 2A and 2B, a combustion products venting system 2 is shown relative to a gas furnace 3. Gas is supplied to a burner 8 through a gas line 9. The manifold pressure of the gas is regulated by a regulator 10. Air which is necessary for combustion is allowed to enter the heat exchanger 7 through an air intake 11. The burner 8 is inserted into a lower portion of the heat exchanger 7. As air within the heat exchanger is heated by the burner 8, the air begins to rise within the heat exchanger until it reaches the air outtake 12. This air, of course, comprises living space air drawn through the intake 11 and products of the combustion of the gas. In the furnace shown in the figure, three separate heat exchangers 7 are shown. The air exiting the air outtakes 12 from the heat exchangers 7 is collected by a collector 13 for venting through the flue 14.
Typically, the natural gas combusted in gas furnaces produces non-toxic combustion products including carbon dioxide, water vapor and nitrogen. However, in instances where the combustion of the natural gas is not complete, harmful products including carbon monoxide, aldahydes and soot are circulated through the interior of the heat exchanger. Thus, if cracks or holes develop in the heat exchanger, these harmful products of combustion may leak from the interior of the heat exchanger into the living space air circulating system 1 so that the harmful products are circulated to the living space of the structure. Thus, it is critically important to detect and locate cracks or holes in heat exchanger as they develop.
Typical heat exchangers are constructed of metals because of their durability and thermal conductivity characteristics. Cracks and holes, however, develop in heat exchangers because of stresses induced by thermal expansion. Corrosive materials which come in contact with the interior and exterior of heat exchangers also weaken the structural integrity of the heat exchanger. Finally, if the furnace experiences improper venting and inadequate supply of combustion air, dirty filters, an improper firing range which produces condensation, or flame impingement, cracks or holes are likely to develop in the heat exchanger. Therefore, regardless of the material from which the heat exchanger is manufactured, the development of cracks and holes in the heat exchanger during its lifetime of service is a real possibility.
Different furnaces are designed to produce different operation pressure levels within the heat exchangers relative to the living space air circulating system. These system fall into two basic categories: naturally draw systems and induced flow systems.
Naturally drawn systems use allow hot air, which rises through a cooler air mass, to draw the products of combustion at the burner through the heat exchanger and out through the flue. This is the same principle by which fireplace chimneys work. As the hot air rises in the flue, a slightly negative pressure is induced within the heat exchanger relative to the air in the heat exchanging section of the living space air circulating system. Thus, even if the holes or cracks exist in the heat exchanger, air will pass into the heat exchanger, rather than out of the heat exchanger, because of the relative low pressure within. The relative negative pressure in naturally drawn systems, however, is small and may be reversed depending on the operating condition of the furnace. For example, when the turban of the living space air circulating system is turned off and does not push air through the heat exchanging section 5, the pressure on the exterior of the heat exchanger 7 may fall below the pressure within the heat exchanger 7. This condition allows air to pass from within the heat exchanger into the living space air circulation system 1. Because harmful combustion products are typically generated during startup, when the turban of the living space air circulating system does not operate, holes and cracks in the heat exchanger must be identified.
Induced flow systems always have greater relative pressure within the heat exchanger. These systems do not have a burner inserted into the heat exchanger. Rather, the burner is located immediately outside the air intake and the flames are blown under pressure into the heat exchanger. Thus, regardless whether the turban of the living space air circulating system is operating, the pressure within the heat exchanger is always greater than the pressure without. It is particularly important to determine the location and size of cracks and holes in the heat exchangers of these systems.
Depending on the furnace system, smaller holes and cracks only allow insignificant amounts of air, well below acceptable safe limits, to pass from the heat exchanger. In fact, many heat exchangers are manufactured with pin point holes and cracks. These are predominantly found at seems between metal component parts. These heat exchangers are typically only used with naturally drawn furnace systems. In these systems, a heat exchanger need not be replaced unless additional holes and cracks develop. Thus, an optimum heat exchanger testing method is one which tests the exchanger under relative pressures similar to its operating pressures. This allows the technician to determine whether the heat exchanger has developed holes and cracks in excess of an acceptable limit for the particular furnace system.
Several heat exchanger testing methods have been use to detect cracks and holes. These include: visual examination, sulfur candles and other odorants, smoke bombs and smoke candles, CFC or HCFC refrigerants, salt sodium sprays, carbon monoxide, titanium tetrachloride, and fluorescent sprays.
The visual examination method simply entails using a mirror and a strong flashlight to inspect the exterior of the heat exchanger. Of course, this method is limited because not all exterior surfaces of the heat exchanger are accessible without removing the heat exchanger from the furnace. Further, it is difficult to determine exactly how much air escapes from the heat exchanger during operation.
Sulfur candles or other odorants are used by introducing the strong odor into the air intake of the heat exchanger. The technician then smells the air in the vicinities of the intake port and plenum of the living space air circulating system 1 to detect a leak. Notwithstanding the harmful effects of the chemical odorants on the heat exchanger, the method is highly unreliable because the location and size of the holes and cracks are not identifiable.
Smoke bombs and smoke candles are inserted into the heat exchanger to detect holes and cracks. The technician closes both the air intake and outtake of the heat exchanger and then ignites the smoke bomb or candle in the interior of the heat exchanger. Holes and cracks are detected by observing smoke escaping from the heat exchanger. This test, however, produces pressures within the heat exchanger significantly dissimilar from pressures observed during normal observation. Thus, smoke is typically forced from the heat exchanger through various small cracks, seams, and even gaskets which do not leak during normal operation. Nearly all heat exchangers are manufactured with very small holes at the seams or gaskets. These are so small that they do not impose a risk leakage under normal conditions. The technician may observe smoke exiting the heat exchanger and recommend replacement of the heat exchanger even where no holes or cracks in the heat exchanger are present.
CFC or HCFC refrigerants are used to detect holes and cracks by placing the refrigerant inside the heat exchanger. A halogen leak detector is then used by the technician to determine if the refrigerant has escaped into the living space air circulating system. Refrigerants, however, produce deadly poisonous gases if combusted. Refrigerants are typically heavier than air and therefore settle in the bottom of the heat exchanger where they reside until they are removed through normal operation of the furnace, i.e., combustion. Thus, the chance of harmful gases being produced is significant. Further, the refrigerant gas method does not allow the technician to determine the location and size of holes and cracks.
Salt sodium sprays are sprayed into the burner flames while the furnace is operated. A propane torch is used by the technician to check the living space air circulating around the exterior of the heat exchanger. Sodium ions are detected when the blue flame of the propane torch turns yellow. While this test may detect very small cracks and leaks, it is impractical because even dust in the air may produce the yellow flame. Thus, dust may be mistaken for the sodium ion. Further, not all exterior surfaces of the heat exchanger are easily accessible for testing with the torch without removing the heat exchanger from the furnace.
Carbon monoxide is used to test for holes and cracks by introducing carbon monoxide into the heat exchanger. Carbon monoxide levels are then observed at the intake port and plenum of the living space air circulating system. However, carbon monoxide is not preferred because dangerous levels of carbon monoxide may inadvertently escape into the living space during the test. Further, like previous methods, the carbon monoxide method does not allow the technician to pinpoint the size and location of cracks and holes in the heat exchanger.
Finally, titanium tetrachloride is used as a fuming or smoking material. The air intake and outtake of the heat exchanger are closed and titanium tetrachloride vapor is introduced into the heat exchanger. Holes and cracks are then identified by observing exiting titanium tetrachloride vapor. Unfortunately, titanium tetrachloride is highly corrosive and considered toxic.
Therefore, there is a need for a device and method for detecting cracks and holes in gas furnace heat exchangers which allows the technician to precisely locate and evaluate cracks or holes under conditions similar to the operating conditions of the heat exchanger.